Patterned tufted articles, such as carpets, have become increasingly popular, particularly in commercial market segments including carpet tiles and hospitality carpets. Carpets having various patterned designs generally can be created by controlling the feeding of yarns, such as through pattern yarn feed attachments, and by shifting the needles of the tufting machine. In forming patterned tufted articles utilizing one or more shifting needle bars, it is important for the needle bars to be shifted or stepped as precisely as possible in order to tuft the yarns or colors of yarns at the tuft or stitch locations required by the pattern being tufted with a necessary sharpness, clarity and accuracy for the formation of the tufted pattern. It also generally is important for the needles to be shifted within as short a time as possible between the time the needles clear the backing and before they re-enter the backing during the downward stroke of their reciprocation cycle. The faster such a shifting movement can be accomplished, the faster the needles can be reciprocated, so as to provide for increased or enhanced production rates. Thus, the speed at which the needle bar or needle bars are shifted generally must be balanced with controlling such shifting movement as accurately as possible to properly present the yarns carried by the needles to their required stitch locations according to the pattern being tufted.
Previously, cam-operated shifters, hydraulic shifters and servomotor-driven shift mechanisms have been used to shift the needle bars of tufting machines. For example, U.S. Pat. No. 5,979,349 of Christman, Jr., et al. discloses a “Tufting Machine with Precision Drive System,” including a roller screw actuator-driven shift mechanism, while U.S. Pat. No. 6,283,055 of Pratt, et al. discloses a tufting machine shifter driven by a linear motor. However, needle bars, especially those required for larger size tufting machines, typically are heavy, creating substantial inertia that must be overcome both in starting and for stopping the shifting movement of a needle bar(s). Overcoming such inertia and accurately and consistently controlling the movement of the needle bar(s), particularly when multiple shift steps or jumps or shifting movements of more than one gauge step are called for in the pattern, can be difficult to accomplish in a very short time span.
Accordingly, it can be seen that a need exists for a tufting machine and a shift mechanism for controlling the shifting of the needles of a tufting machine that addresses the foregoing and other related and unrelated problems in the art.